The present invention relates to a developing roller which is, in an electrophotographic system or a electrostatic recording system such as a copying machine or a printer, used for supplying a one-component developer on an image forming body represented by a photosensitive drum or belt for holding an electrostatic latent image, or a sheet of paper, transparency, or photographic paper, thereby allowing a visible image to be formed on a surface of the image forming body. It specifically related; and; to a developing apparatus using the developing roller. In particular, the present invention relates to a developing roller capable of obtaining a high quality image without the occurrence of an uneven image density and/or a background fogged image and of attaining a high gradient, and to a developing apparatus using the developing roller.
In an electrophotographic system or an electrostatic recording system such as a copying machine or a printer, as a developing method for supplying a one-component developer on a latent image support such as a photosensitive drum for supporting a latent image, to stick the developer on the latent image formed on a surface of the latent image support, thereby visualizing the latent image, a pressing type developing method has been known, for example, from U.S. Pat. Nos. 3,152,012 and 3,731,146. This method is advantageous in facilitating simplification and miniaturization of the developing apparatus and also facilitating coloring of toner because of no need of using a magnetic material.
In this pressing type developing method, a developing roller for supporting toner (generally, composed of a non-magnetic one-component developer) is brought in contact with a latent image support such as a photosensitive drum for supporting an electrostatic latent image, to stick the toner on the latent image formed on the latent image support, thereby allowing the latent image to be developed. Accordingly, the developing roller used for this method is required to be formed of an elastic body having a conductivity.
The pressing type developing method will be more clearly described with reference to FIG. 2. Referring to FIG. 2, a developing roller 1 is disposed between a toner applying roller 4 for supplying toner and a photosensitive drum 5 for supporting an electrostatic latent image. The developing roller 1, photosensitive drum 5, and toner applying roller 4 are rotated in the direction indicated by an arrow in FIG. 2, so that toner 6 is supplied on a surface of the developing roller 1 by the toner applying roller 4. The toner 6 thus supplied is then adjusted into a uniform thin layer by a layer forming blade 7. In such a state, by rotating the developing roller 1 while bringing it in contact with the photosensitive drum 5, the toner 1 formed into the thin layer on the developing roller 1 is deposited to and adheres on a latent image formed on the photosensitive drum 5, whereby the latent image is visualized. In this figure, reference numeral 8 indicates a transfer portion at which a toner image is transferred on a recording medium such as a paper sheet; and 9 is a cleaning portion at which the toner remaining on the surface of the photosensitive drum 5 after transfer of the toner image is removed by a cleaning blade 10.
In this case, the developing roller 1 is required to be rotated while maintaining close-contact with the photosensitive drum 5. Consequently, as shown in FIG. 1, the developing roller 1 has a structure such an elastic layer 3, which is made from an elastic rubber such as a silicon rubber, NBR or EPDM, or a urethane foam mixed with a conductive agent for giving a conductivity thereto, is formed around an outer periphery of a shaft 2 made from a highly conductive material such as a metal.
Another type developing method has also be proposed in Japanese Patent Laid-open No. Sho 58-116559, wherein a non-magnetic toner formed in a thin layer is supported on a surface of a developing sleeve disposed in proximity to, that. is, not in contact with a latent image support and is flied and supplied on the latent image support, whereby a latent image on the latent image support is developed with the toner. Further, there have been known a developing method using a latent image support being not formed in a drum shape but in a belt shape, and a developing method of forming an image by directly supplying toner from a developing roller onto a recording medium such as a sheet of paper, transparency, or photographic paper. Even in each of these developing methods, a developing roller similar to that described above can be used.
On the other hand, recently, in addition to the so-called monochromatic developing performance necessary for printing only characters, a high gradient is also required to meet a demand for realizing a higher image quality necessary for a photographic image. Consequently, it has been required to design a developing roller allowing a developed amount by toner to be moderately changed depending on a running developing bias. To obtain such a characteristic, it may be considered to design a developing roller in which a resistance of the roller is made high and also a surface resistance of the roller is made high.
In the case where the roller resistance is made high, however, a developing bias is reduced on the basis of a voltage drop due to the roller resistance, failing to obtain a sufficiently developed amount, and at present, such a problem cannot be solved by the related art.